Importance of non-ideal effects for peeling-ballooning stability thresholds in spherical tokamaks*

It is shown that non-ideal physics can crucially alter peeling-ballooning (PB) stability limits in spherical torus (ST) configurations. Emphasis is placed on identifying conditions under which these extended-MHD effects become important. Ideal-MHD has been successful in predicting PB stability thresholds [and associated edge-localized modes (ELMs)] in conventional aspect ratio tokamaks. Although ideal-MHD finds agreement with observations of ELMs in STs in some cases, such as some ballooning-limited cases in MAST, its success in describing stability limits in STs is far more limited than in conventional aspect ratio tokamaks. We employ the extended-MHD code M3D-C1 to investigate macroscopic edge stability in ELMing and ELM-free discharges in NSTX and MAST. It is found that PB growth rates in ELMing NSTX discharges tend to exhibit a strong scaling with resistivity, especially in kink-peeling limited regimes, while in ELM-free scenarios and in the considered MAST discharge this happens to a lesser extent. We present analysis to determine under what conditions non-ideal MHD effects are required to accurately describe PB stability. These results constitute a valuable basis for the development of a predictive model for ELMs in STs, which is an important step towards a compact fusion power plant.